Internet Engineering Task Force (IETF) A. Clark
Request for Comments: 7005 Telchemy
Category: Standards Track V. Singh
ISSN: 2070-1721 Aalto University
Q. Wu
Huawei
September 2013
RTP Control Protocol (RTCP) Extended Report (XR) Blockfor De-Jitter Buffer Metric Reporting
Abstract
This document defines an RTP Control Protocol (RTCP) Extended Report
(XR) block that allows the reporting of de-jitter buffer metrics for
a range of RTP applications.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7005.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Clark, et al. Standards Track [Page 1]

RFC 7005 RTCP XR Jitter Buffer September 20131. Introduction1.1. De-Jitter Buffer Metrics Block
This document defines a new block type to augment those defined in
[RFC3611] for use in a range of RTP applications.
The new block type provides information on de-jitter buffer
configuration and performance.
The metric belongs to the class of transport-related end-system
metrics defined in [RFC6792].
Instances of this metrics block refer by synchronization source
(SSRC) to the separate auxiliary Measurement Information Block
[RFC6776], which contains information such as the SSRC of the
measured stream, and RTP sequence numbers and time intervals
indicating the span of the report.
1.2. RTCP and RTCP Extended Reports
The use of RTCP for reporting is defined in [RFC3550]. [RFC3611]
defines an extensible structure for reporting using an RTCP Extended
Report (XR). This document defines a new Extended Report block for
use with [RFC3550] and [RFC3611].
1.3. Performance Metrics Framework
"Guidelines for Considering New Performance Metric Development"
[RFC6390] provides guidance on the definition and specification of
performance metrics. "Guidelines for Use of the RTP Monitoring
Framework" [RFC6792] provides guidance on the reporting block format
using RTCP XR. Metrics described in this document are in accordance
with the guidelines in [RFC6390]and [RFC6792].
1.4. Applicability
Real-time applications employ a de-jitter buffer [RFC5481] to absorb
jitter introduced on the path from source to destination. These
metrics are used to report how the de-jitter buffer at the receiving
end of the RTP stream behaves as a result of jitter in the network;
they are applicable to a range of RTP applications.
These metrics correspond to terminal-related factors that affect
real-time application quality and are useful for providing a better
end-user quality of experience (QoE) when these terminal-related
factors are used as inputs to calculate QoE metrics [QMB].
Clark, et al. Standards Track [Page 3]

RFC 7005 RTCP XR Jitter Buffer September 20132. Standards Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. De-Jitter Buffer Operation
A de-jitter buffer is required to absorb delay variation in the
network delivery of media packets. A de-jitter buffer works by
holding media data for a period of time after it is received and
before it is played out. Packets that arrive early are held in the
de-jitter buffer longer. If packets arrive too early, they may be
discarded if there is no available de-jitter buffer space. If
packets are delayed excessively by the network, they may be discarded
if they miss their playout time.
The de-jitter buffer can be considered a time window with the early
edge aligned with the delay corresponding to the earliest arriving
packet and the late edge representing the maximum permissible delay
before a late arriving packet would be discarded. The delay applied
to packets that arrive on time or at their expected arrival time is
known as the nominal delay, and this is equivalent to the time
difference/buffer size difference between the insertion point of the
on-time packets and the point at which the packets are read out.
The reference for the expected arrival time may be, for example, the
first packet in the session or the running average delay. If all
packets arrived at their expected arrival time, then every packet
would be held in the de-jitter buffer exactly the nominal delay.
The de-jitter buffer maximum delay is the delay that is applied to
the earliest arriving packet that is not discarded and corresponds to
the early edge of the de-jitter buffer time window.
3.1. Idealized De-Jitter Buffer
In practice, de-jitter buffer implementations vary considerably;
however, they should behave in a manner conceptually consistent with
an idealized de-jitter buffer, which is described as follows:
Clark, et al. Standards Track [Page 4]

RFC 7005 RTCP XR Jitter Buffer September 2013
(i) Receive the first packet and delay playout by D ms. Keep the
RTP timestamp (TS) and receive time as a reference.
RTP TS[1]
receive time[1]
Assume that both are normalized in ticks (there are 10,000
ticks in a millisecond).
(ii) Receive the next packet.
(iii) Calculate r = RTP TS[n] - RTP TS[1] and t = receive time[n] -
receive time[1]. If r == t, then the packet arrived on time.
If r < t, then the packet arrived late, and if r > t, then the
packet arrived early.
(iv) Delay playout of packet by D + (r-t).
(v) Go back to (ii).
Note that this idealized implementation assumes that the sender's RTP
clock is synchronized to the clock in the receiver, which is used to
timestamp packet arrivals. If there is no such inherent
synchronization, the system may need to use an adaptive de-jitter
buffer or other techniques to ensure reliable reception.
3.2. Fixed De-Jitter Buffer
A fixed de-jitter buffer lacks provision to track the condition of
the network and has a fixed size, and packets leaving the de-jitter
buffer have a constant delay. For fixed de-jitter buffer
implementation, the nominal delay is set to a constant value
corresponding to the packets that arrive at their expected arrival
time, while the maximum delay is set to a constant value
corresponding to the fixed size of the de-jitter buffer.
3.3. Adaptive De-Jitter Buffer
An adaptive de-jitter buffer can adapt to the change in the network's
delay and has variable size or variable delay. It allows the nominal
delay to be set to a low value initially to minimize user perceived
delay; however, it can automatically extend the late edge (and
possibly also retract the early edge) of a buffer window if a
significant proportion of the packets are arriving late (and hence
being discarded).
Clark, et al. Standards Track [Page 5]

RFC 7005 RTCP XR Jitter Buffer September 2013
I=10: Interval Duration - the reported value applies to the
most recent measurement interval duration between successive
metrics reports.
I=11: Cumulative Duration - the reported value applies to the
accumulation period characteristic of cumulative measurements.
In this document, de-jitter buffer metrics can only be sampled and
cannot be measured over definite intervals. Also, the value I=00
is reserved for future use. Senders MUST NOT use the values I=00,
I=10, or I=11. If a block is received with I=00, I=10, or I=11,
the receiver MUST discard the block.
Jitter Buffer Configuration (C): 1 bit
This field is used to identify the de-jitter buffer method in use
at the receiver, according to the following code:
0 = Fixed de-jitter buffer
1 = Adaptive de-jitter buffer
Reserved (resv): 5 bits
These bits are reserved. They MUST be set to zero by senders and
ignored by receivers (see [RFC6709], Section 4.2).
Block Length: 16 bits
The length of this report block in 32-bit words, minus one, in
accordance with the definition in [RFC3611]. This field MUST be
set to 3 to match the fixed length of the report block.
SSRC of Source: 32 bits
As defined in Section 4.1 of [RFC3611].
De-jitter buffer nominal delay (DJB nominal): 16 bits
This is the current nominal de-jitter buffer delay (in
milliseconds) that corresponds to the nominal de-jitter buffer
delay for packets that arrive exactly on time. It is calculated
based on the time spent in the de-jitter buffer for the packet
that arrives exactly on time. This parameter MUST be provided for
both fixed and adaptive de-jitter buffer implementations.
Clark, et al. Standards Track [Page 7]

RFC 7005 RTCP XR Jitter Buffer September 2013
The measured value is an unsigned value. If the measured value
exceeds 0xFFFD, the value 0xFFFE MUST be reported to indicate an
over-range measurement. If the measurement is unavailable, the
value 0xFFFF MUST be reported.
De-jitter buffer maximum delay (DJB maximum): 16 bits
This is the current maximum de-jitter buffer delay (in
milliseconds) that corresponds to the earliest arriving packet
that would not be discarded. It is calculated based on the time
spent in the de-jitter buffer for the earliest arriving packet.
In simple queue implementations, this may correspond to the size
of the de-jitter buffer. In adaptive de-jitter buffer
implementations, this value may vary dynamically. This parameter
MUST be provided for both fixed and adaptive de-jitter buffer
implementations.
The measured value is an unsigned value. If the measured value
exceeds 0xFFFD, the value 0xFFFE MUST be reported to indicate an
over-range measurement. If the measurement is unavailable, the
value 0xFFFF MUST be reported.
De-jitter buffer high-water mark (DJB high-water mark): 16 bits
This is the highest value of the de-jitter buffer nominal delay
(in milliseconds) that occurred at any time during the reporting
interval. This parameter MUST be provided for adaptive de-jitter
buffer implementations, and its value MUST be set to DJB maximum
for fixed de-jitter buffer implementations.
The measured value is an unsigned value. If the measured value
exceeds 0xFFFD, the value 0xFFFE MUST be reported to indicate an
over-range measurement. If the measurement is unavailable, the
value 0xFFFF MUST be reported.
De-jitter buffer low-water mark (DJB low-water mark): 16 bits
This is the lowest value of the de-jitter buffer nominal delay (in
milliseconds) that occurred at any time during the reporting
interval. This parameter MUST be provided for adaptive de-jitter
buffer implementations, and its value MUST be set to DJB maximum
for fixed de-jitter buffer implementations.
The measured value is an unsigned value. If the measured value
exceeds 0xFFFD, the value 0xFFFE MUST be reported to indicate an
over-range measurement. If the measurement is unavailable, the
value 0xFFFF MUST be reported.
Clark, et al. Standards Track [Page 8]